1. Field of the Invention
This invention relates to a pneumatic radial tire comprising a belt comprised of at least two belt layers and a belt reinforcing layer for the belt embedded therein with a reinforcing element extending in a circumferential direction while bending in wavy form.
2. Description of Related Art
Recently, the profile flattening of the tire proceeds with the lowering of a floor in a vehicle. As the tire becomes flattened, the outward growth of tire size in a tread portion in a radial direction becomes larger in the inflation under an internal pressure and hence separation failure is easily caused at an end of a belt to lower the tire durability.
In order to solve this problem, there has been proposed a pneumatic tire as described in JP-A-2-208101. This tire comprises a radial carcass toroidally extending between a pair of bead portions, a belt disposed at an outside of the radial carcass in a radial direction of the tire and comprised of at least two belt layers each containing many reinforcing cords embedded therein, the cords of which layers are crossed with each other with respect to an equatorial plane of the tire, a belt reinforcing member disposed at an inside of the belt in the radial direction and comprised of at least one ply having a width narrower than that of the belt and containing plural reinforcing elements embedded therein so as to extend in a circumferential direction of the tire while bending in form of wave or zigzag, and a tread rubber arranged at an outside of the belt in the radial direction.
In such a pneumatic tire, the belt reinforcing member controls the outward growth of tire size of a tread portion, particularly a shoulder portion in the radial direction during the inflation of an internal air pressure to a certain extent, whereby separation failure at a widthwise outer end portion of the belt is controlled to improve the belt durability.
However, the growth of the tire diameter in the tread portion increases with the flattening of the tire, so that the width of the belt reinforcing member should be made wide in accordance of the flattening degree (i.e. aspect ratio) of the tire. In this connection, when the width of the belt reinforcing member in the above pneumatic tire is further widened in accordance with the more flattening of the tire recently demanded (aspect ratio of not more than 0.7), there is caused a problem that the occurrence of belt end separation can not sufficiently be suppressed and particularly the separation failure is created at the widthwise outer end portion of the belt reinforcing member.
The inventors have made various studies with respect to the separation failure of the belt in the above pneumatic tire provided with the belt reinforcing member and have obtained the following knowledge. That is, when the tire rides on protrusions such as stones and the like scattered on road surface at a middle region between the equatorial plane and the tread end, during the running of the tire, the belt reinforcing member is pushed inward in the radial direction to form a dent portion. The degree of this dent portion is sharper in the widthwise direction than in the circumferential direction. For this end, when the tire rides over the protrusion, a large axial shearing strain is caused between the belt layers constituting the belt, and particularly a largest axial shearing strain is caused at an end of the belt layer, which brings about the occurrence of the belt end separation as mentioned above.
The inventors have made analyses with respect to the deformation of the belt as mentioned above and confirmed two causes as mentioned below. Firstly, the reinforcing elements of the belt layers constituting the belt are crossed at a small inclination angle with respect to the equatorial plane of the tire, usually within a range of 15-30xc2x0 for effectively controlling the growth of tire diameter in the tread portion, while the reinforcing elements embedded in the belt reinforcing member extend in parallel to the equatorial plane of the tire as a whole as mentioned above, so that the bending rigidity of the belt and the belt reinforcing member in the widthwise direction of the tire as a whole is fairly low. Secondly, the belt reinforcing member bears a greater part of circumferential tension produced in the tire in the inflation of the internal air pressure because the reinforcing elements embedded in the belt reinforcing member extend in parallel to the equatorial plane, so that the rigidity rapidly lowers at an outer end of the belt reinforcing member in the axial direction, and hence when the belt reinforcing member is subjected to a force from the protrusion as mentioned above, it acts as a single deformable plate and concavely deforms at the axially outer end as a fulcrum (or fixed point).
Furthermore, the inventors have confirmed the following fact. That is, since the belt in a ground contact region is deformed so as to be flat and the reinforcing elements contained therein are inclined toward the side of the equatorial plane so as to extend in the circumferential direction, rubber surrounding the belt is dragged in the circumferential direction by such a deformation of the belt and also the dragged quantity of rubber becomes large near to the widthwise outer end of the belt. In the above tire, since the belt reinforcing member having a width narrower than that of the belt is arranged adjacent to the belt, rubber located near to the widthwise outer end of the belt reinforcing member is also dragged by the widthwise outer end portion of the belt and largely deformed. However, even if the belt reinforcing member is deformed so as to be flat together with the belt, the reinforcing element in the belt-reinforcing member extend in the circumferential direction, they are not further stretched in the circumferential direction and hence a great strain is produced in rubber near to the widthwise outer end of the belt reinforcing member, and also such a strain is repeatedly caused every the rotation of the tire. As a result, the separation failure is prematurely caused in the vicinity of the widthwise outer end of the belt reinforcing member.
The inventors have made further studies based on the above knowledge and found that the occurrence of separation failure can be controlled by arranging a belt protection member satisfying a given condition in addition to the belt reinforcing member, or by specifying a relation between widthwise outer ends of the belt and the belt reinforcing member.
According to a first aspect of the invention, there is the provision of a pneumatic radial tire comprising a carcass toroidally extending between a pair of bead portions and comprised of at least one rubberized ply containing a plurality of cords arranged substantially in a radial direction, a belt arranged at an outside of the carcass in the radial direction and comprised of at least two belt layers containing plural cords embedded therein, the cords of which layers being crossed with each other with respect to an equatorial plane of the tire, a belt reinforcing member arranged adjacent to the belt and comprised of at least one belt reinforcing layer containing a reinforcing element embedded therein and extending in a circumferential direction while bending in form of wave or zigzag, and a tread rubber arranged on an outside of the belt in the radial direction, in which a belt protection member containing many reinforcing elements embedded at an inclination angle with respect to the equatorial plane larger than that of the cord in the belt layer is disposed in the neighborhood of the belt and a widthwise outer end of the belt protection member is located outward from a widthwise outer end of the belt reinforcing member in an axial direction of the tire.
Thus, when the belt protection member having a width wider than that of the belt reinforcing member and having a high bending rigidity in the widthwise direction is disposed in the neighbor hood of the belt, the bending rigidity in the widthwise direction of the belt and the belt reinforcing member as a whole is increased by the belt protection member, while the fulcrum of deformation when being subjected to external force from a protrusion scattered on road surface is shifted outward from the widthwise outer end of the belt reinforcing member to the widthwise outer end of the belt protection member in the axial direction, so that the degree of dent portion produced in the belt becomes slow-moving and shearing strain produced between the belt layers in the widthwise direction becomes small and hence the occurrence of belt end separation is controlled.
In a preferable embodiment of the invention, the widthwise outer end of the belt protection member is located inward from the widthwise outer end of a belt layer having a minimum width among the belt layers in the widthwise direction. Thus, shearing strain produced between the belt and the belt protection member can be controlled to effectively prevent the separation failure therebetween.
In another preferable embodiment of the invention, the belt protection member is disposed on an outside of an outermost belt layer among the belt layers. Thus, the belt can effectively be protected from an external damage such as cut or the like, while the bending rigidity in the widthwise direction of the belt and the belt reinforcing member as a whole can effectively be increased because the belt protection member is far away from a neutral plane of the recess deformation.
In the other preferable embodiment of the invention, an inclining direction of the reinforcing element in the belt protection member is the same as an inclining direction of the cord in the belt layer adjacent to the belt protection member. Because, if the inclining direction of the reinforcing element in the belt protection member is opposite to the inclining direction of the cord in the belt layer, a great shearing strain is produced between the belt protection member and the belt layer to bring about the occurrence of separation failure.
In a further preferable embodiment of the invention, the inclination angle of the reinforcing element in the belt protection member is not less than 30xc2x0, preferably 40-70xc2x0. In this case, the bending rigidity in the widthwise direction of the belt reinforcing member and the belt as a whole can more effectively be increased while preventing the separation failure.
In a still further preferable embodiment of the invention, a cushion rubber layer is arranged between the widthwise outer end portion of the belt protection member and the widthwise outer end portion of the belt layer adjacent to the belt protection member, whereby shearing strain between the widthwise outer end portion of the belt protection member and the belt layer can effectively be mitigated to more control the separation failure therebetween.
According to a second aspect of the invention, there is the provision of a pneumatic radial tire comprising a carcass toroidally extending between a pair of bead portions and comprised of at least one rubberized ply containing a plurality of cords arranged substantially in a radial direction, a belt arranged at an outside of the carcass in the radial direction and comprised of at least two belt layers containing plural cords embedded therein, the cords of which layers being crossed with each other with respect to an equatorial plane of the tire, a belt reinforcing member arranged adjacent to the belt and comprised of at least one belt reinforcing layer containing a reinforcing element embedded therein and extending in a circumferential direction while bending in form of wave or zigzag, and a tread rubber arranged on an outside of the belt in the radial direction, in which a widthwise outer end of a belt reinforcing layer having a widest width among the belt reinforcing layers is located outward from a widthwise outer end of a belt layer having a widest width in an axial direction of the tire.
Thus, when the widthwise outer end of a belt reinforcing layer having a widest width among the belt reinforcing layers is located outward from the widthwise outer end of a belt layer having a widest width in a widthwise direction of the tire, rubber in the vicinity of the widthwise outer end portion of the widest-width belt reinforcing layer is separated away from the widthwise outer end of the widest-width belt layer largely deforming in the circumferential direction, so that it is not affected by such a deformation and hence the separation failure in the vicinity of the widthwise outer end of the widest-width belt reinforcing layer can effectively be prevented. In this case, the belt reinforcing member may be arranged inward or outward from the belt in the radial direction or between the belt layers constituting the belt.
In a preferable embodiment of the invention, when a maximum width of the carcass is L, the widthwise outer end of the widest-width belt reinforcing layer is located between a point P separated outward from the equatorial plane S by 0.375 times of L in the widthwise direction and a point Q separated outward from the equatorial plane S by 0.45 times of L in the widthwise direction. In this case, the outward growth of tire size in the tread portion can effectively be controlled and the distribution thereof can be uniformized while preventing the separation failure in the widthwise outer end of the widest-width belt reinforcing layer.
In another preferable embodiment of the invention, when the width of the widest-width belt layer is N, the widthwise outer end of the widest-width belt reinforcing layer is located outward from a point U separated by 0.05 times of N from the widthwise outer end of the widest-width belt layer in the widthwise direction. In this case, the deformation of the widest-width belt layer in the circumferential direction is hardly caused in the vicinity of the widthwise outer end of the widest-width belt reinforcing layer and hence the separation failure in such a widthwise outer end is more controlled.